Referring to FIG. 1, in the prior art, the flat cable generally includes the signal unit set 10 formed by arranging a plurality of signal units 101 on the same plane. The upper and lower surfaces of the signal unit set 10 are covered with insulating films 11, 12. The upper and lower insulating films 11, 12 are laterally extended by a small length on both sides of the signal unit set, and are bonded to each other. Recently, based on the considerations of width of the wire and aesthetics, the width of the small extension is generally less than 1 mm. During use, it is likely to cause the insulating film to be easily peeled off, resulting in short-circuit and other consequences, affecting the use of the product.